This invention relates to a dropout corrector which uses an estimate of the video being processed to determine the presence or absence of a dropout.
Video transmission channels may be subject to dropouts, which are short-duration decreases in signal level which may result in a total loss of signal. In a transmission channel including a broadcast portion, such a reduction in signal level might be described as "fading", and might be due to multipath effects attributable to atmospheric conditions or movable reflectors such as aircraft. In the case of a transmission channel including a tape recorder, the losses of signal or dropouts result chiefly from tape defects. A tape defect might be an area without magnetic oxide or it might be a lump of oxide which lifts the tape away from the recording or playback head in the region just before and just after the lump.
In analog video tape recording, small-scale variations in the response of the tape may produce noisy video. To avoid this, the video signal ordinarily frequency-modulates a radio frequency (RF) carrier signal which is then recorded onto the tape. The small variations in amplitude on playback are then stripped off by a limiter associated with the FM demodulator and the resulting video is substantially noise free. In such systems, a dropout reduces the amplitude of the RF carrier. Upon playback, the reduced signal strength of the carrier introduces noise into the demodulated video. This noise is introduced at least in part because the carrier signal level is so low that it cannot be limited by the limiters of the FM demodulator.
Prior art dropout correctors have used schemes which take advantage of the redundancy of the video signal by substituting for the video signal which occurs at the time of the dropout a signal derived in some fashion from the adjacent data. For example, in one prior art scheme, the presence of a dropout is detected by sensing the reduction of the magnitude of the RF carrier transduced by the pickup head which occurs during a dropout and when a preselected lower amplitude limit is reached, a controlled switch couples data from the preceding horizontal line as a substitute for the data occurring during the dropout as established by the RF signal amplitude criterion.
Digital video recording systems are presently being considered as substitutes for the analog recorders heretofore used. Digital video recording systems record series of pulses which collectively represent numbers which in turn represent the amplitude of the video signal from moment to moment (pixels). If these pulses were used to frequency-modulate an RF carrier, a dropout might be detected by operating on the RF carrier amplitude. It is desirable to directly record onto the tape the pulses representative of the video signal. When the pulses are directly recorded, a dropout causes the pulse amplitude to decrease. Since an RF carrier is not used in such a direct recording system, a continuous signal is not available which can be compared with a threshold to determine the presence of a dropout. Indeed, a complete dropout in a directly recorded system might be indistinguishable upon playback from an inter-pulse period of the digital signal.
A tape dropout will ordinarily occur over a tape length which would normally be occupied by a portion of the digital video signal representing several adjacent picture elements or pixels. Copending application Ser. No. 139,237, filed Apr. 11, 1980 for Clemens, et al. and entitled "TAPE FORMAT TO FACILITATE ERROR CONCEALMENT AND APPARATUS FOR RECORDING AND/OR REPLAYING SAME" describes a system in which the pulse groups of the digital signal which represent individual pixels are spread throughout the tape as recorded in such a manner that it is unlikely that a single dropout will affect adjacent pixel information. Consequently, each dropout rather than causing loss of several adjacent pixels of information causes a spread-out pattern of erroneous pixels each of which is surrounded by correct pixels.
Copending application Ser. No. 170,811, filed July 21, 1980 for Reitmeier and entitled "TWO DIMENSIONAL ADAPTIVE DROPOUT COMPENSATOR AND CHROMA INVERTER" describes an adaptive estimator which generates signals which may be substituted for dropped-out information. The estimator uses delay lines to simultaneously compare the rate of change of the magnitude of the video signal represented by the digital signal being processed in several spatial (in the plane of the raster) directions in the vicinity of the dropped-out pixel. The rate-of-change is determined in the vertical direction by taking the difference of the values of the pixels immediately above and below the raster position of the dropped-out pixels. The horizontal rate-of-change is determined by taking the difference in the values of the pixels to the right and to the left of the dropped-out pixel. Similarly, differences of pixels on two diagonals of the raster about the dropped-out pixel establish the rate of change of the signal in the diagonal directions. A logic circuit determines which rate of change is the least. An average signal interpolated between the two adjacent pixels in the direction adaptively selected as having the least rate of change is then substituted for the dropped-out pixel. If the raster display is, for example, a horizontal transition (upper half of the raster is white, lower half is black) and the dropped-out pixel is on the black side of the boundary line, the vertical and diagonal differences about the dropped-out pixel will represent a full video amplitude excursion, i.e., white minus black. In the horizontal direction, the difference will be zero. Zero results in the horizontal direction because the adjacent pixels in the horizontal direction will both have the same value, namely black. Since the least video amplitude difference in the example is in the horizontal direction, an average of the pixels to the right and to the left of the dropped-out pixel is substituted for the dropped-out pixel. The average in the horizontal direction about the dropped-out pixel will be the average of a black level and a black level, which is a black level. Consequently, the estimate is a black pixel. For the example given, this estimate is a perfect reproduction of the dropped-out pixel. A similar result would have been achieved in the case of a vertical transition, because the least-difference criterion would have required that an average in the vertical directon be substituted for the dropped-out pixel. This adaptive scheme produces an extremely high-quality estimate of the value of the dropped-out signal. If the digital video signals are directly recorded onto the tape or other recording medium, the amplitude of the digital pulses can be monitored to determine the presence of a dropout, and the described estimator can be enabled to correct the signal for dropouts to provide broadcast-quality performance.
In the case of a directly-recorded digital signal, no RF carrier is available which can be used to provide an indication of the existence of a dropout. A dropout corrector is desired which may be used with directly recorded video signals and which may be used to cause an estimator such as that described above to substitute a high-quality estimate of the video for the dropped-out sample.